Agree, especially for mid-size or small towns, only

their own welcome centers would have local maps.

If we have a trip that is a few months down the road, I normally would look up the official chamber of commerce, or tourism sites of the cities we may pass. Then I would try to email them to ask for a information packet - such as maps, transportation maps, places to visit, etc. 85% of the time they would send the requested information.